Since the end of the Cold War, the question “Whither NATO—and why?” has come up regularly, especially in the United States. This is not an idle question nor one that can simply be dismissed. If anything, it is remarkable that the North Atlantic Treaty Organization still exists a quarter-century after the key reason for its creation—the widely shared perception of a political, strategic, and military threat from the Soviet Union—ceased to exist. To be sure, there is now renewed challenge from the Soviet Union’s principal successor state, the Russian Federation. From the beginning of the 1990s, however, until the Russian seizure of Crimea in 2014, a span of nearly 25 years, the argument could have been made that there was no need for continuing the Western alliance that did so much to contain Soviet power and the Warsaw Pact and that played a significant role in the dissolution of both. Many people did argue just this point, both in the United States and elsewhere, but they were never in the majority (or at least they never prevailed in public and parliamentary debate). The reasons for NATO’s continued existence are important to understand, including to provide a basis for considering its future and, more precisely, the tasks it should be asked to perform and its very character as an alliance of sovereign states spanning the two sides of the Atlantic.1